(A) Field of the Invention
The present invention relates to a developing apparatus using a magnetic developer, in which a latent image on a latent image-bearing material being rotated is developed by a magnetic developer layer formed and delivered by utilizing the difference of the rotation speed between a developing sleeve and a magnet roll rotatably arranged on the inner side of the developing sleeve.
(B) Description of the Prior Art
In a conventional developing apparatus for developing an electrostatic latent image on a photosensitive plate with a magnetic developer, a developing sleeve and/or a magnet roll is rotated to deliver the magnetic developer in the form of a layer to the developing zone. In order to obtain a desirable thickness of the magnetic developer layer for the development, a doctor blade is arranged on an opening formed on the delivery side of a magnetic developer vessel and the distance between the doctor blade and the developing sleeve is adjusted from about 0.2 mm to about 0.5 mm according to the desirable thickness of the magnetic developer layer. However, since the flow of the magnetic developer is dammed up by the doctor blade, a large pressure is imposed on the magnetic developer, agglomeration is readily caused, the opening between the doctor blade and the developing sleeve is clogged with the agglomerates and it is sometimes impossible to form a magnetic developer layer having a uniform thickness consistently stable. This phenomenon is called "blocking".
It is known that in connection of the inhibition of occurrence of this blocking, the delivery method in which only the magnet roll is rotated or the magnet roll is mainly rotated and the developing sleeve is subsidiarily rotated is advantageous over the delivery method in which only the developing sleeve is rotated or the developing sleeve is mainly rotated and the magnet roll is subsidiarily rotated. This delivery method in which the magnet roll is mainly rotated and the developing sleeve is subsidiarily rotated is disclosed, for example, in the specification of U.S. Pat. No. 4,267,248.
According to this known delivery method, as shown in FIG. 1, a magnetic developer 10 is attached onto a cylindrical non-magnetic sleeve 14 rotatably mounted to confront a photosensitive drum 12 bearing an electrostatic latent image on the surface thereof by a magnetic attracting force of a magnet roll 16 having a plurality of magnetic poles on the surface thereof, which is rotatably arranged within the non-magnetic sleeve 14, and by the relative rotation between the non-magnetic sleeve 14 and the magnet roll 16, the magnetic developer 10 is delivered to bring the magnetic toner 10 into contact with the surface of the photosensitive drum 12. In this method, the non-magnetic sleeve 14 and the magnet roll 16 are rotated in the same direction and the rotational speed of the magnet roll 16 is made sufficiently larger than the number of rotations of the non-magnetic sleeve 14, whereby the magnetic developer 10 as a whole is delivered in a direction opposite to the rotation direction of the non-magnetic sleeve 14 and the magnet roll 16, and by adjusting the gap between the photosensitive drum 12 and the non-magnetic sleeve 14 to a value smaller than the thickness of the magnetic developer 10 on the non-magnetic sleeve 14, a developer reservoir 18 extended in a substantially uniform thickness in the circumferential direction of the non-magnetic sleeve 14 is formed in the vicinity of the point where the surface of the photosensitive drum 12 confronts the non-magnetic sleeve 14. The development is started at the point where the surface of the photosensitive drum 12 first falls in contact with the developer reservoir 18.
In this method, the gap 28 on the delivery side of the magnetic developer tank 20 is set by the doctor blade and the amount supplied of the magnetic developer 10 is regulated by this gap 28. Since the delivery of the magnetic developer 10 is mainly performed by rotation of the megnet roll 16 in this method, occurrence of blocking is effectively prevented. However, since the regulation of the gap 28 is still performed by the doctor blade, it is difficult to prevent occurrence of blocking over a long period of time. Furthermore, in this method, in the case where a large quantity of the magnetic developer 10 is stored in the magnetic developer tank 20, the magnetic developer 10 recovered from the gap 24 on the recovery side is not delivered to the gap 28 on the delivery side and is gradually accumulated in the gap 24 on the recovery side.
Furthermore, a large quantity of the magnetic developer 10 is reversely delivered from the gap 24 on the recovery side with rotation of the non-magnetic sleeve 14, and finally, the magnetic developer 10 overflows from the magnetic developer tank 20 even to the developing zone, resulting in degradation of the quality of the developed image and falling of the magnetic developer 10.